ABSTRACT Cellular adhesion, migration and communication mechanisms have been recognized as crucial elements regulating the fate and function of leukocytes and their participation in immune responses. As such, the direct visalization of cell-cell and cell-pathogen interactions and the fine structural features that influence these events is central to our understanding of how the immune system responds to antigen challenge. The Intravital Microscopy Core (Core B) was conceived based on a stated need of program investigators to draw on the experience of its key personnel to provide expertise, infrastructure and resources to design and execute in vivo imaging studies of immune responses to viral infections. The Specific Aims of the Intravital Microscopy Core are as follows: Aim 1: To provide access and expertise in the conduct of specific experiments utilizing both epifluorescence video intravital microscopy (IVM) and multiphoton intravital microscopy (MP-IVM) to record intra- and extravascular blood cell adhesion and migration as well as cell-cell and cell- pathogen interactions in as many as six dimensions (i.e. space, time, color, fluorescence intensity). Aim 2: To develop, validate, and provide ready-to-use retro- and lentiviral vectors and recombinant HIV strains to express fluorescent protein-based reporter constructs in immune cells for use in MP-IVM studies. Aim 3: To offer a custom-designed environment with appropriate biosafety conditions for the generation, housing and intravital microscopy analysis of virally infected mice. Aim 4: To execute and analyze intravital microscopy-based adhesion and migration studies in murine tissues. Currently available imaging models include: bladder; bone marrow; cremaster muscle; ear skin; knee joint; liver; popliteal and inguinal lymph node; small intestine, including mesentery, Peyer's patches and lamina propria; spinal cord; spleen; thymus and uterus. Aim 5: To provide assistance in the planning, execution and analysis of blood cell homing in vivo employing defined models of inflammation and infectious diseases. Aim 6: To provide computational resources for the storage, processing and in-depth analysis of digital imaging data. Services provided by Core B will allow participating investigators to explore innate and adaptive immune cell behavior at a resolution ranging from single cell organelles to intact animals in an environment dedicated to the study of replicating mammalian pathogens.